LONDON (Reuters) - Giving paracetamol to babies to prevent fever after routine vaccinations may reduce the effect of the shots themselves, Czech scientists said on Friday.

While the paracetamol, known as acetaminophen in the United States, generally does limit post-vaccination fever, it also reduces the child's response to some of the vaccine antigens, according to a study in The Lancet journal.

Mothers in developed countries whose babies have a series of routine vaccinations at around the age three months are often told by medical staff to give paracetamol to try to cut the risk of fever or febrile convulsions.

But Roman Prymula of the Czech University of Defence said his study showed that giving so-called anti-pyretic medicines like paracetamol after vaccinations should "no longer be routinely recommended without careful weighing of the expected benefits and risks."

Paracetamol, sold under the brand Tylenol in the U.S. market, is used to relieve fever, minor aches and pain, and is given as a liquid suspension to children -- often sold in Britain as Calpol.

Prymula and colleagues studied around 260 babies in the Czech Republic who were going for routine infant immunisations.

Around half were given three paracetamol doses every 6 to 8 hours in the first 24 hours after vaccination, while the other half were given no paracetamol.

The vaccinations were the shots generally offered to children in developed nations as protection against pneumococcal disease, Haemophilus influenzae type b (Hib), diphtheria, tetanus, whooping cough, hepatitis B, polio and rotavirus.

The effect of some of the vaccines -- in particular Hib, diphtheria, tetanus and pneumococcal -- was reduced in the paracetamol group, who had lower concentrations of antibodies.

The researchers said they were not sure why the vaccine effect was dampened, but said one theory was that it was due to paracetamol's effect in preventing inflammation. They said more research was needed to investigate the response further.